1. Field of the Invention
This invention relates to lens mountings for use in auto-focus cameras, and more particularly to a device for controlling the range of rotation of a distance adjusting ring.
2. Description of the Prior Art
The ordinary auto-focus cameras generally operate in such a manner that the distance to an object to be photographed is measured by the range finder, and then, based on the output signal therefrom, a distance adjusting ring of the lens mounting is rotated to move the focusing lens member to an in-focus position. For some combinations of the value of the object distance and the occasional position of the focusing lens member, however, a problem arises that it takes a considerably long time to reach the in-focus condition.
FIG. 1 illustrates an example of the conventional zoom lens mounting used in the auto-focus camera, and its cross-section along A--A line of FIG. 1 is shown in FIG. 2. Here, a guide tube 1 has axially elongated slots 1a and 1b, and a mount portion 1c for coupling with a camera body (not shown), and is fixedly secured to an outer barrel 4 by a screw fastener 3. A cam sleeve 2 is rotatably fitted on the outer diameter of the guide tube 1. An electric motor 5 is mounted on the outer barrel 4. Movably fitted in the inner diameter of the guide tube 1 is a sleeve 6 having a helicoid screw cut in the inner surface thereof and meshing with a helicoid screw 7a of a lens cell 7 holding a lens component I. A cam follower pin 8a extends radially outwardly of the lens cell 7 through the first slot 1a of the guide tube 1 into a camming groove 2a of the cam sleeve 2. Another lens cell 9 holding a second lens component II is also movably fitted in the inner diameter of the guide tube 1, and has a cam follower pin 8b radially outwardly extending through the second slot 1b into a second camming groove 2b of the cam sleeve 2. A retainer ring 10 having a ball race surface is fixedly secured to the guide tube 1 by a screw fastener 10a and retains a ring 11 having a ball race surface. A distance adjusting ring 12 between the combined ring of the retainer ring 10 and the ring 11 and the outer barrel 4 has a ball race surface 12a facing through balls at the race surfaces of the rings 10 and 11. An outward pointing radial projection 12b of the distance adjusting ring 12 lies in a circumferential inner recess portion 4a of the outer barrel 4. The outer periphery of the distance adjusting ring 12 is provided with a racked portion 12c drivenly connected through a pinion 14 to the output shaft of the motor 4. A radial projection 7b of the first lens cell 7 extends into an axially elongated groove 12d formed in the inner surface of the distance adjusting ring 12. The outer barrel 4 is drivingly connected through a pin 15 to the cam sleeve 2.
Then, when the outer barrel is manually turned about the optical axis, the lens components I and II are axially moved in differential relation to effect variation of the image magnification. To effect automatic focusing, a range finder 17 evaluates the object distance. Responsive to the output of the range finder 17, a control circuit 18 supplies electrical power from a battery 19 to rotate the motor 5 along with the distance adjusting ring 12 through the pinion 14. Rotation of the distance adjusting ring 12 is transmitted through the helicoid screw connection 7a to axial movement of the first lens component I.
With the range finder 17 of the generally employed or TTL type, however, as is well known, there are some combinations of the position of the object and the position of the lens component I for which it is impossible to detect the in-focus condition. Even with other types such as that employing the supersonic generator, it is impossible to precisely measure the object distance over the entire range from infinity to the minimum.
To improve this drawback of the TTL type, there has been a previous proposal that when no signal is detected, the motor is reversed and continues to be energized until the signal is obtained. According to this proposal, if the motor 5 rotates in an opposite direction to that in which the lens component I is moved to bring the image into focus, the projected portion 12b of the distance adjusting ring 12 will abut against either one of the radial shoulders 4c and 4d at the respective ends of the recessed portion 4a of the outer barrel 4 so that rotation of the distance adjusting ring 12 is limited. It is also known to use two switches at the shoulders 4c and 4d of which the outputs control the direction of rotation of the motor 5, as, for example, disclosed in Japanese Laid-Open Patent Application No. SHO 56-94334.
In consideration of achieving good manageability of the photographic lens, the angle of rotation of the distance adjusting ring is generally taken at a considerably large value as indicated at .theta. in FIG. 2. If the focusing lens member is moved in the opposite direction to that in which the in-focus signal is detected in a least time, as has been mentioned above, it will take a very long time to arrive at the in-focus position. Even with the other types than the TTL one, when the angle of rotation of the distance adjusting ring 12 is large, a considerably long time is necessary to reach the in-focus condition.